The dynamic behavior of pre and post capillary resistance elements and their effect on transcapillary fluid movement will be studied in a variety of physiologic and pharmacologic conditions, using the isolated gracilis muscle and the vascularly isolated, innervated hind limb preparations. The focus of this project is on the timing, and nature of the changes in specific microvascular tone which occur as a result of physiological or pharmacological influences and their composite effect on capillary function. In order to conduct this inquiry in a critical manner, it is first essential that the methods most prominently used in this research area, namely, the Pappenheimer zero flow extrapolation method, and the Mellander estimate of capillary filtraton coefficient, be critically evaluated. This will be done using the Friedman multiparametric experimental design in which transcapillary fluid movement will be assessed gravimetrically and oncotically and protein leakage by direct tissue monitoring of 125I-albumin accumulation.